No U-turn on LEZ: Constitutional Court suspends Brussels’ LEZ postponement

The standstill principle: the constitutional foundation of the case

The standstill principle is enshrined in Article 23 of the Belgian Constitution, which guarantees the right to the protection of health and to a healthy environment. The principle prohibits legislators from substantially reducing, without adequate justification, the level of protection previously afforded to these rights. In other words, once a certain standard of protection is established by law, any substantial legislative regression must be justified by compelling reasons and must be proportionate to the objectives pursued.

The Court has consistently interpreted this obligation as a safeguard against arbitrary or unjustified rollbacks of fundamental rights. It requires a careful balancing of interests and a rigorous assessment of whether less restrictive alternatives could achieve the same policy goals.

Arguments of the applicants

The applicants, composed of civil society organizations and individuals, argued that this postponement constituted a significant and unjustified regression in the level of protection for health and the environment. Notably, one of the applicants was a child suffering from chronic asthma and allergies, whose medical condition is directly aggravated by air pollution. They pointed to scientific and official reports demonstrating that the LEZ had already reduced NOx emissions by 36%, black carbon by 65%, and NO2 concentrations by 30% on major roads since its introduction. The 2025 phase was expected to halve fine particle emissions from traffic, a benefit now delayed. The applicants also provided medical evidence showing that children and individuals with chronic respiratory diseases are particularly harmed by air pollution, and that the delay would cause irreversible health damage, especially among these vulnerable groups. They further stated that the Brussels-Capital Region had failed to provide concrete evidence that the most polluting vehicles were predominantly owned by low-income or vulnerable groups, and that less restrictive alternatives - such as targeted exemptions or financial aid for vehicle replacement - were available but not adopted.

The Constitutional Court’s reasoning and decision

The Court began by establishing the relevant baseline for protection: the stricter LEZ standards scheduled to take effect on 1 January 2025, as set out in the 2022 government decree. The Court rejected the Region’s argument that the postponement merely maintained the status quo, holding that the legal standard is determined by the most recent, duly adopted regulation, not by what has or has not yet been implemented in practice. By retroactively authorizing the continued circulation of vehicles that would have been banned under the 2025 phase, the ordinance of 21 March 2025 constituted a significant regression in the level of protection of the right to health and a healthy environment, as guaranteed by article 23 of the Constitution.

The Court then turned to the question of justification. It emphasized that the standstill principle does not impose an absolute prohibition on regression, but any significant reduction in protection must be justified by reasonable and proportionate grounds. The Court examined the Region’s argument of social and economic concerns related to the impact on low-income households and certain professionals and found that they were insufficiently substantiated. More particularly, the Court found that at this stage, there is no concrete evidence showing that the most polluting vehicles were predominantly owned by lower-income groups. It emphasized that low-income households are also the one most vulnerable to air pollution and the resulting health problems. In this respect, the postponement introduced by the contested measure is general and not targeted. According to the Court’s decision, it allows a large number of people to continue emitting pollutants, which will primarily affect the very people the measure is supposed to protect.

The Court found that the risk of grave and irreparable harm was established. The Court accepted the applicants’ arguments, as it recognized that the health impacts of air pollution, especially on vulnerable individuals such as the child applicant with chronic asthma, are well-documented and often irreversible. Even a temporary delay in implementing stricter LEZ standards could result in significant and irreparable harm to public health. The general postponement, therefore, failed the test of proportionality.